1. Field of the Invention
This invention relates to the field of plasma etching of tungsten and more specifically to the etching of a tungsten layer in the fabrication of a semiconductor device.
2. Prior Art
In the fabrication of high-density semiconductor devices, refractory metals are increasingly used as low-resistivity interconnect and contact barrier. These refractory metals provide an alternative to conventional Aluminum-based metallization. When used in very large scale intergration (VLSI) involving metal-oxide semiconductor (MOS) technology. Typically, these metals are deposited by various deposition techniques well-known in the prior art. Then these metals are patterned and etched using various prior art techniques. However, one of the more popular methods of etching is provided by plasma etching techniques. An advantage of plasma etching refractory metals is disclosed in an article titled by A. Picard and G. Turban; Plasma Etching of Refractory Metals (W,MO,TA) and Silicon in SF.sub.6 and SF.sub.6 -O.sub.2, An Analysis of the Reaction Products; Plasma Chemistry and Plasma Processing; Vol. 5, No. 4, 1985; pp 333-351.
One particular refractory metal, namely tungsten (W), has received favorable response for use in fabricating VLSI devices. Tungsten provides a high electromigration resistance and offers better step coverage and thermal stability when used as a metallization layer in the manufacturing of VLSI devices. Etching of tungsten is typically performed by the use of a dry plasma etching technique using CF.sub.4, SF.sub.6 or oxygen impregnated gas mixtures, such as CF.sub.4 plus O.sub.2, or SF.sub.6 plus O.sub.2. Although plasma etching is normally preferred, reactive ion etching also performs well, such as when CF.sub.3 Br-O.sub.2 -He gas mixture is used. Etching of tungsten using fluorinated gases or the CF.sub.3 Br mixture are documented in an article by E. Degenkolb, M. Burba, S. Henck, M. Tabasky, E. Jungbluth; entitled Selective Dry Etching of Tungsten for VLSI Metalization; Abstract Number 244; Journal of the Electro-chemical Society; 167 Society Meeting Volume 85-1, 1985; p 353; and also in an article by C. C. Tang, D. W. Hess; titled Tungsten Etching and CF.sub.4 and SF.sub.6 Discharges; Journal of the Electrochemical Society; January 1984; pp 115-120.
There are several disadvantages related to etching tungsten by the prior arts methods described. The tungsten etch rate for CF.sub.3 Br-O.sub.2 -He plasma is too slow to be cost-effective. Also, the reactive ion etcher is more costly than a parallel plate plasma etcher. Further, fluorinated gases of the plasmas of CF.sub.4, SF.sub.6, CF.sub.4 plus O.sub.2 or SF.sub.6 plus O.sub.2 resulted in mask undercut while the tungsten was being etched. Still further, the range of oxygen concentration in SF.sub.6 plus O.sub.2 to provide anisotropic tungsten etch requires such small quantities of oxygen which prevent good process control.
It is appreciated that what is needed is a method to anisotropically etch a tungsten metal film in a conventional plasma etcher without undercutting the conventional photo resist mask, and yet, provide close process control over the etching cycle.